What do the display codes mean?

The display in the center of your instrument pod is called the BDI, or Battery discharge Indicator.  However, the BDI actually gives 4 categories of information. 
They are:
-Battery Discharge Indicator
-MPH Speed Indicator: when in motion, shows actual MPH on stock 12" tires.
-Accumulated Miles: Odometer
-Error Code Display

The BDI will tell you the amount of charge left in your battery bank, in the form of a remaining percentage, from 100% down.

The Miles Per Hour will be displayed while in motion, and will alternate with the BDI.  The MPH is only accurate with the stock 12" tires, unless reprogrammed for a specific tire size difference, since it is calculated based upon RPM of the stock tire.

Accumulated miles are shown when the vehicle is not in motion, the brake is on, and the key is on.

Error Code Display is when the vehicle is not in motion, and there is an issue preventing proper operation.  Codes are displayed with a "dash" then a 2 digit code.  There are only 5 "owner remedy" codes.  Any other codes require dealer service, according to the GEM manual.  Let's cover those 5 one at a time.

-04 Parking Brake engaged at key ON position.  (Can also mean brake safety switch engaged/broken)
-06 Pedal depressed while direction switch is in neutral.  (You're not in gear, or switch has failed)
-11 Pedal depressed at key ON.  (Turn the key OFF, remove foot from pedal, turn key ON)
-15 Low voltage on battery bank.  (Batteries need charge before further operation possible)
-16 High voltage on battery pack.  (Turn on LIGHTS for several moments, then retry)

Details:

-04 The Parking Brake Safety Switch is installed to prevent damage to the motor that can be caused if the motor is under load while the brake is on.  The motor can overheat, and be permanently damaged as a result.  There is a switch installed under the parking brake boot that can, as discussed in an earlier article, cause this particular code because of loose wires at the terminal connections, a failed safety switch or broken wires prior to the electric engage circuit located under the dash.  To test for this, put the key in the ON position, drive switch to FWD, foot off the brake and accelerator.  Now, grasp the parking brake handle, depress the lock button, and lower the handle.  If when you fully lower the parking brake to the disengaged position, you do NOT hear an audible "clack" under the dash, then the safety switch has a flaw.  Begin with the contacts below the boot, and work your way forward, from least expensive to fix, to most.

-05 Obviously if the direction switch is in NEUTRAL, the vehicle will not go anywhere.  Remove your foot from the accelerator, move the switch to FWD or RVS, and try again.  If the vehicle still does not move, it may be a loose wire, a bad switch or the worst possible item, the motor itself.  Even so, a new motor is less than $1,000, and takes approximately 25 minutes to change out.  (See article on motor upgrade)

-11 A lot of care was put into preventing motors from being damaged.  If you have this code, the computer control is sensing that your foot is on the accelerator before the key was in the ON position.  If your foot is not on the accelerator, and you are still seeing this code when you first turn the key on, check your pedal to make sure it is fully up by manually lifting it with your hand.  If it is not, there may be a problem with the return spring, or debris stuck along the moving surfaces.  If neither of these is the case, you must consider that the accelerator control unit has failed.

-15 Just like rechargeable flashlights, cordless drills and RV batteries, there comes a point during use of your vehicle when there is not enough charge in your battery bank to effectively move your vehicle.  Although it is not typically known to cause battery damage, it is not ever recommended to completely drain your battery bank before recharging is provided.  This code should come up early enough while you are driving, for you to have enough charge remaining to get to an outlet, but chances are if you were not paying attention to the BDI in the first place to know what percentage you had left, then you probably have not seen this code.  Make it a habit to "top off" your batteries whenever it is convenient.  If you get this code when you are just beginning to use your vehicle for the day, and you have had your vehicle plugged in, first, check your outlet.  If your outlet has power, then check the charger by plugging your vehicle in.  You should hear a few "beeps", followed by the sound of a small fan, and a flashing red light above the BDI.  If one or all of these does not happen, unplug your vehicle from the outlet and contact your dealer, as it appears that your charger is no longer working, and must be replaced.  If you DO hear and see all of these things, then you must let the batteries fully charge.  There may have been an issue such as the plug not being put fully in, a bad cord, etc.

-16 This may happen if your vehicle battery bank has a charge that is higher than the maximum recommended voltage for your vehicles motor.  Although this is not common, it can happen.  To prevent any damage to the motor, the manufacturer recommends that you "spill off" the extra power by turning on the headlights for moments to minutes to lower the battery level to a safe level.  IF THIS HAPPENS CONTINUALLY, then you have an issue with your charger,  Please consult a qualified GEM technician as soon as possible, to prevent any damage to your batteries.

Again, any codes that are not covered here is due to the manufacturer preference that such codes be brought to the attention of a qualified GEM technician.

I hope this is useful for you.  Any comments or questions, just drop them here!